Generally filtering process means a process in which raw water including suspended solids flows into a filter and filtered water is produced by eliminating suspended solids from the raw water. That is, in the filtering process, the suspended solids in the raw water are captured at pores formed in a filtering layer which is formed of filter media by complicated mechanism such as a screening, a precipitation, an inertial collision, an interception, an absorption, cohesion, etc., and thereby clean water, that is, filtered water, is discharged.
As the filtering process is continuously performed, the pores formed in the filtering layer are filled with the suspended solids so that the filtering resistance is increased. When the filtering resistance exceeds a certain degree, the filtering process cannot be performed. At this time, a washing process performed by supplying a clean washing water and air is needed in order to eliminating the suspended solids filling the pores.
Organic and inorganic solids are suspended in the urban sewage and industrial waste water. If the urban sewage and industrial waste water is discharged into the natural waters such as rivers, lakes, marshes, sea, etc, the turbidity of the water is increased and dissolved oxygen is decreased by the organic materials, that is, the natural waters are polluted.
Meanwhile, in the case of filtering the continuously flowing water which needs mass filtering, the filter may be classified into a continuous cross flow gravity filter and a continuous down flow gravity filter or a continuous down flow pressure filter depending on the flow of water.
The continuous cross flow gravity filter includes an Aqua Disk Filter and a Micro Disk Filter. In the Aqua Disk Filter which is an American type, the raw water flows into a water tank by gravity and is filtered by using a carpet type filtering cloth. In the Micro Disk Filter which is a Swedish type, the raw water passes through a close net connected to a variable low-speed rotary center drum from inside to outside and be filtered.
The continuous cross flow gravity filter has advantages as follows. The filtered water has high quality, the filter doesn't need a large site for installation, and it is easy to repair the filter because the filter is manufactured by the respective unit.
But the continuous cross flow gravity filter has disadvantages as follows. It is difficult to cope with the change of the quality or amount of influent raw water, the filtering efficiency is increased but the amount of filtering capacity is decreased if the number of rotation of the rotary drum is decreased or the frequency of washing a disk is decreased, the filter element is damaged by high pressure while washing each disk so that the life of the filter is decreased, and when repairs or replacement of filter media is performed, the operation of filter needs to be stopped.
The continuous down flow gravity filter includes a sand media gravity filter in which raw water flows in to the upper part of a filtering layer, passes through the filtering layer downward and then flows out to a collecting basin placed below. The said filtering layer comprises mainly sand and the life of the filter media is about 5 years.
The sand media gravity filter has advantages as follows. Blockade of the filter media due to mud ball, etc. doesn't occur. And the headloss is small so that filtration by natural flow of water is possible.
The sand media gravity filter has disadvantages as follows. The filter needs a large site for installation, large investment is needed, the growth of algae is hastened by the sun light, the amount of the filter media (i.e., sand) is large so that it is difficult to exchange the filter media, and the operation needs to be stopped in order to exchange the filter.
Therefore, in the filtration of a large amount of continuously flowing water, i.e., discharged waste water, the filter of the prior art still has problems to be improved.